Decoupling the simultaneous effects of NO2−, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

•·The determining role on N2O production transited from NO2− to FNA especially beyond 0.015 mg HNO2-N/L.•·pH affected N2O pathway regulation but not the overall N2O production rate.•·Elevated pH promoted NN while decreased pH promoted ND pathway.•·NO2− promoted ND N2O production pathway, while high FNA inhibited it.•·The susceptibility of NO production to FNA lagged behind that of N2O.